The invention generally relates to a method and system to optimise soft handover gain in networks such as CDMA (Code Division Multiple Access) networks, or wideband code division multiple access (WCDMA) systems such as the universal mobile telecommunication system (UMTS).
In CDMA Radio Access Network Optimisation several methods are used for tuning the network performance. Soft Handover is one of the important optimisation areas because it has significant influence on system quality, capacity and coverage. Soft Handover may provide, for uplink, Rx diversity, separate decoding at each NodeB, selection combining at RNC (based on frame reliability indicator), and for downlink, double signal power, double interference, improved fading statistics. Different DL power control commands are possible, or a combination of soft values.
Soft-handover (SHO) means that a user equipment (UE) may be connected to two or more base stations (BS) when moving from one cell to another. In general, application of SHO makes it possible to have seamless (transparent) handover and improved coverage. However, SHO requires a higher average transmit power level from BS's in the network, also know as the SHO overhead. The SHO overhead is in general a function of the handover parameters Window13 Add and Window13 Drop.
For optimising the Soft Handover (SHO) usually a parameter “Soft Handover (SHO) Overhead” is measured and tuned. This value is easy to measure. The metric, i.e. measured value of “SHO Overhead” is tuned to a (arbitrary) specific target value. This target value is determined based on uniform traffic distributions and provides decent network performance for such cases. The target for the “SHO Overhead” in the tuning is also often set to a value which is acceptable in terms of the allowed capacity for the backbone network, e.g. BTS-RNC (Base Transceiver Station, Radio Network Controller) links. The importance of the “Soft Handover Overhead” metric is to make sure that the overhead in terms of the number of connections per cell is not excessive.